A broadstrike involves a coin hit while it's not in the retaining collar. It sandwiches outward, and having nothing to retain its shape and size, is always larger than the normal coin should be. This isn't what you have.
What you have is an odd form of die fatigue progression (die wear) that happened commonly in the 80s while the Mint was still experimenting with getting the zinc planchet thing straightened out. This form of die wear is very common for the era of coins yours is in.